Field effects of High-T_c Oxide Superconductors expected from their characterisctics of low carrier densities and short coherent lengths are hopefully applicable to superconducting three terminal devices. To achieve actual devices, however, a lot of difficulties, i. e fabrication of high quality ultra thin ETSC films with the thickness of several nm and development of insulating layr with a large dielectric constant still remains. From the another point of view, tilt grain boundaries not only behave as Josephson junctions but also have a large field sensitiviy because of local deficiency of charge carriers due to oxygen deficiency in the boundaries.In this research, we have investigated the electric field effects on artificial grain boundaries of YBCO films and obtained following results.・The carrier density of the grain boundary was evaluated to be 2-5*10^<20>cm^<-3> from the measurement of the electric field effect on a normal conducting artificial grain boundary adjoining two superc
… Moreonducting YBCO grains. The carrier density was one drder lower than that in inner grains so that large electric field effects were expected. This is the first result indicating the usefulness of grain boudaries as field effect channels.・As electric field effects with respect to AC and DC Josephson effects on the grain boundaries behaving as Josephson junctions, electric field modulations of both critical current, I_c and self resonance frequency were observed. For former effect, it was shown that the field effect in I_c was qualititavely explained by a charge carrier modulation in grain boundary junctions on the basis of the SNS model. For later one, the variations of the resonance frequencies were attributed to changes in the effective dielectric constants, epsilon^GB_/d reflecting the field strength dependence ofthe dielectric constant of STO used as substrates. The electric field modulation of epsilon^GB_/d is possibly applicable to the phase velocity control of electromagnetic waves transmitting in the grain boundaries in the case of Flux Flow Oscilator (FFO). Less